In telecommunication systems, the use of in-band tones for signaling is common. For example, touch-tone phones commonly use a tone that is formed of two component tones to send digits in a signaling scheme called dual-tone multi-frequency (DTMF). Dual tone signals are also used in customer premise equipment alert signaling (CAS), subscriber alert signaling (SAS) and as visual screen list editing (VSLE) tones.
However, in conventional tone relays, leakage through the voice path can result in the false detection of tones on the far end of the network. Therefore, to avoid tone leakage into the voice path, conventional systems may queue (but not release) the voice samples in the encoder system. For example, the voice encoder may be delayed by the maximum time that is necessary to guarantee the detection of a tone (about 40 msec in the case of a DTMF tone). Such a delay would result in zero leakage through the voice path but may negatively affect voice quality because it adds 40 msec of delay to the system. In addition, in memory limited systems, 40 msec of extra buffering is required in the ingress direction.
Alternatively, some systems pre-detect tones through a combination of tone decisions and state machine processing. Typically the incoming signal is queued if a tone is predetected. The tone detector typically makes a decision (i.e. is there a tone present) every five-fifteen msec. If the detection was false (invalid), the voice samples are ultimately released, otherwise they are discarded. This manifests itself as jitter when tones are falsely pre-detected. The extra jitter introduced due to false early tone detections may result in lost packet, frame repeats, and/or increased system delay.
Therefore, it would be advantageous to provide a method and apparatus for the detection and removal of frequency tones in voice signals without introducing significant jitter or delay into the signal.